…ixes #494.

TypeScript provides the ReadonlySet and ReadonlyMap types which match
their normal counterparts but without any methods that mutate their data.
This change makes Immutable<Map> and Immutable<Set> map to those types
directly.

Worth noting that Map extends ReadonlyMap so we only need to check for
one to know what the resulting type is, the same goes for Set and
ReadonlySet.

This also removes the intermediate Set/Map types since TypeScript no
longer seems to need those (likely since TypeScript 3.7, see #448).

…d ReadonlySet.

… improved-types